The present invention generally pertains to video information signal processing and is particularly directed to storage and retrieval of video information in a slow scan television system.
Slow scan techniques were developed to accommodate high data rate sources to narrow band communication channels. Slow scan techniques have had particular application to the transmission of video information, such as a picture developed as a television type raster scan frame, over voice grade communication channels at a data rate that can be accommodated in the limited bandwidth available. The time necessary to transmit sufficient information for a video presentation of acceptable quality requires on the order of 8 to 32 seconds per frame. Such slow frame rates are incompatible with conventional television monitors because the phosphors utilized on the face of the display tube do not have sufficient persistence to stay illuminated for a full frame.
One type of slow scan television system utilizes a specially designed slow scan display tube incorporating long persistence phosphors. In such a tube, the illumination produced by the electron beam produces an after glow of the long persistence phosphors that permits viewing of a complete frame in relatively low ambient light conditions. The face of the tube is the only storage for the received image and it is therefore necessary to have a continuously repeating image for longer display. On such tubes, the writing line, or bright portion of the tube, makes it difficult to view the remainder of the tube. Further, the 8 second delay in production of a new picture makes it difficult to adjust the system for proper contrast and other picture variables.
The disadvantages associated with the use of long persistence phosphors has led to the development of other slow scan television systems that incorporate storage for a complete frame of video information separate from the display tube. A slow scan image is written into a memory until a completed picture is developed. The completed image may then be retrieved from the memory as many times as is desired at a fast scan rate and displayed on a conventional television monitor. However, such systems are not capable of simultaneously monitoring the stored image and storing/retrieving slow scan information.
Various systems have been proposed incorporating a second memory unit so that fast scan monitoring and simultaneous slow scan read/write functions may be obtained. However, such systems are excessively expensive for many applications and increase the complexity of the system thereby degrading reliability and operational flexibility.